1. Field of the Invention
The present invention is directed to materials for filling dental root canals.
2. Description of Related Art
Dental root canal treatment generally involves three stages: shaping, cleaning and obturation (generally involving filling and sealing). The ultimate objective of root canal treatment is to eliminate the infection inside the dental root system and to tightly seal or obturate, in three dimensions (3-D), the tiny openings at the end of the root canal, (referred in the profession as an apex). Failure to completely seal the apex or the root canal in 3-D leads to micro-leakage, which will lead to future bacteria colonization inside the root canal system, and re-infection and possible loss of the tooth. Micro-leakage is the most common cause of tooth failure.
Heretofore, root canal treatment processes involve placement of a root canal filling or sealing point or cone in a prepared root canal to plug the root canal, ideally in a manner to eliminate micro-leakage. However, the existing filling points and the process of application thereof do not lend themselves well to provide a good seal of the root canal apex. Heretofore, root canal filling points are formed of a filling material that is shaped into slender cones each having a small taper angle (e.g., 5-10 degrees).
The most commonly used root canal filling material for many years is a biocompatible dry latex compound commonly called Gutta Percha (which comprises trans-polyisoprene). Zinc oxide and Barium Sulfate are fillers included in the compound. Zinc Oxide is present in an amount that provides the function of major inert filler inside dental Gutta Percha root canal filling material to improve dimensional stability. The Barium Sulfate is present in an amount that provides radio opacity of the filled root canal. Heretofore, the Zinc Oxide filler is of size on the order of greater than 50 microns. Gutta Percha molecule chains serve as a matrix supporting the inert fillers. Gutta Percha may also refer to balata and isoprene. FIG. 1 is an embodiment of the chemical structure of Gutta Percha, having the chemical composition of 1,4-trans-polyisoprene (TPI).
The following thermal mechanical characteristics of dental Gutta Percha were observed:
1. There are three crystal phases of Gutta Percha: alpha, beta and amorphous. Gutta Percha undergoes phase transitions with temperature changes. Specifically, when temperature is raised to between 42 to 49 degree Celsius, alpha phase will transition to beta phase. If temperature is raised to 53 to 59 degree Celsius, it transitions to amorphous phase. When heated Gutta Percha material is cooled at an extremely slow rate, the material will recrystallize to the phase that is stable at the lower temperature (e.g., at the alpha phase at room temperature). Otherwise if cooling is not at a slow enough rate, the material will cool to a phase corresponding to a higher temperature (e.g., at the beta phase). Natural Gutta Percha is mostly in the alpha phase, and dental Gutta Percha is mostly in the beta phase due to the heat treatment the Gutta Percha material was subject to during the manufacturing process of dental Gutta Percha filling points.
2. During a root canal treatment procedure, temperature is necessarily raised to soften the Gutta Percha material to fill the root canal. The temperature is raised above the threshold to cause the material to transition to the amorphous phase, so as to flow the material to fill the root canal. Thereafter, given it is difficult to control the cooling rate, the Gutta Percha material will normally cool to the beta phase, resulting in shrinkage in volume after the root canal treatment procedure, which leads to potential micro-leakage problem.
3. Given dental Gutta Percha material is a rather poor thermal conductor, and further coupled with heat dissipation by the patient's jaws, it has been known that heat is not effectively conducted to soften the material beyond about 3.0 mm from a reasonable heat source. This will prevent having softened Gutta Percha forming a perfectly fitted plug at the root canal apex area to provide a 3-D seal to prevent micro-leakage.
Currently, most dentists use a heated probe to soften and to push dental Gutta Percha cones/points into root canals, by a technique known as Warm Gutta Percha technique. Most of the time, the probe temperature is set at significantly above the Gutta Percha phase change temperature threshold. This will induce crystal phase change; therefore shrinkage will occur upon cooling. One way to counter this is to use the probe to apply pressure on the Gutta Percha material in the root canal while the material is being cooled. In order to have a perfect 3-D seal at root canal apex area, dentists like to push the heat probe as deep into the root canal as possible to soften the distal tip of Gutta Percha cone. Because the dental root structure at the apex area is rather thin and to avoid burning tissue outside the apex area by the heated probe, dentists are not supposed to push the heat tip within 5 mm from the root canal apex. Given that dental Gutta Percha can only conduct heat wave no more than 3 mm at its best, the tip portion of the Gutta Percha cone is not softened to seal the apex portion of the root canal. Therefore a perfect 3-D seal in not achieved.
There is a system on the market today which could introduce pre-heated/pre-softened Gutta Percha into apex area via a solid carrier. The problem with this approach is that this solid carrier has to permanently stay inside the root canal system as part of the final seal. When root canal retreatment is needed, this solid carrier may not be effectively removed to regain access to the apex area of the root canal.
Attempting to satisfy dentists' clinical needs to have a perfect 3-D seal at root canal apex, manufacturers today are trying to lower Gutta Percha's viscosity (using Mooney indexing machine for measurement), so as to increase Gutta Percha's flowability for better 3-D molding when softened with heat. During the manufacturing process, to lower Gutta Percha's viscosity and increase its flowability, the dental Gutta Percha compound is subject to much higher heat treatment for extended time period. Gutta Percha molecule chain is a low temperature latex material. When it is subject to very high temperature, it is degraded, carbonated, and evaporated. When this happens, dental Gutta Percha compound loses its matrix and becomes a very dry and brittle mass, which will make potential micro leakage even a bigger possibility. Some manufacturers try to add a significant dose of mineral wax at the end of heat treatment to facilitate bonding of inert fillers together inside the dental Gutta Percha compound. Problems with using excessive wax are: (1) Wax evaporates even easier under heat and will leave voids in the root canal filling material. (2) Wax oxidizes very easily to cause the entire dental Gutta Percha compound to break down with much shorter shelf-life.
It can be seen that the current root canal treatment procedures involve complex and challenging steps, which may still result in micro-leakage. It would be desirable to develop an improved root canal filling material to facilitate the root canal treatment procedure to improve treatment success rate.